#!/usr/bin/python3
import argparse
import re
import sys


def main():
    p = argparse.ArgumentParser(description="Paragraph grep")
    p.add_argument('regexp', type=re.compile)
    p.add_argument('infile', nargs='*')
    opts = p.parse_args()

    for infile in (map(open, opts.infile) if opts.infile else [sys.stdin]):
        with infile:            # XXX closes sys.stdin, which should be OK
            paragrep(opts.regexp, infile, sys.stdout)

def paragrep(regexp, infile, outfile):
    "Write paragraphs from infile matching regexp to outfile."
    for para in paragraphs(infile):
        if any(regexp.search(s) for s in para): outfile.writelines(para)

def paragraphs(lines):
    para = []
    for line in lines:
        para.append(line)
        if not line.strip():
            yield para
            para = []

    yield para

if __name__ == '__main__':
    main()